Jason Horowitz / The Washington Post

VATICAN CITY — Shortly after arriving in Rome with his boss, dark horse papal contender Cardinal Sean O’Malley, Terrence Donilon received an email. It was from his brother Tom, the national security adviser to President Barack Obama.

“How’s it going?” the top White House official asked.

The answer: astonishingly well.

In the days leading up to today’s conclave, O’Malley has built up momentum. The archbishop of Boston, a baritone-voiced Capuchin Franciscan who prefers the order’s humble brown cassock, O’Malley has earned increasing attention from Vatican reporters, and, in the estimation of many experts, bypassed New York Archbishop Timothy Dolan as the prelate with the best, if still distant, shot of becoming the first American leader of the Roman Catholic Church. That unexpected buzz has also raised a remarkable possibility for Terry Donilon, O’Malley’s communications director and cabinet member.

One Donilon brother “working for the most powerful man on the planet and the other one could work for the most powerful religious leader on the planet?” mused Terry on Saturday, dressed in blue baseball cap and polo shirt in a cafe by the Vatican. “Yeah that’s kind of an interesting storyline.”

For O’Malley, 68, it could also be a complicated one. The church has traditionally excluded Americans from papal consideration for fear of allotting too much influence and might to the world’s superpower. Changes in geopolitics and the demands of the church have softened that unofficial ban, but it’s unclear what it will mean for a potential symbol of world peace to have a close associate who is also the brother of an architect of Obama’s foreign policy.

And while reports have emerged that Tom Donilon is expected to step down this year (“I’m still fully engaged,” he said), there is also a third Don-ilon brother, Mike. A longtime political aide to Vice President Joe Biden, he would probably play a major role in a Biden 2016 presidential bid. (Perhaps more conveniently, the brothers also have a sister, Donna, a nurse who is widely considered a saint.)

“This is an extraordinary circumstance, not one that we’ve ever thought about,” Tom said.

In the unlikely event that any of the cardinal electors are concerned by the Donilon family ties, it is also worth noting that Terry Donilon, 52, has already shown his willingness to stand up to the Obama administration. Donilon attacked provisions in the administration’s Affordable Care Act that required health insurance plans to offer contraceptives and access to other procedures anathema to the Catholic Church. He argued that recent concessions were inadequate.

“The way the administration handled that was poor,” Donilon said, adding, “My brothers have their life and their careers, and I’ve had my life and my careers and if they intersect at times because of issues, so be it.”

The Donilon children grew up in a solidly Irish section of Providence, R.I., where St. Michael’s Parish acted as a center of gravity. Faith, Terry said, was the “fabric of who we were.” He and his brothers served as altar boys, and the family lived in a Dutch Colonial home where they all learned instruments: Donna and Tom the piano; Mike the guitar. Terry, a singer, proved the most musical and later sang with the Gregorian Concert Choir in St. Peter’s and at the funeral of Tom’s mother-in-law, where he noticed Biden mouthing the Latin words of the Panis Angelicus hymn along with him.

“He’s a much more public person than Michael and I,” Tom, 57, said.

The other tie that has bound the entire Donilon family is politics. Their father, a member of the school board, was involved in local races, and their mother — a “little tiny Irish lady” and “poor lady’s Rose Kennedy” — had “no problem speaking her mind to the priest,” Terry said.

“Coming out of Vatican II, my mother was deeply involved in the parish,” added Tom, referring to the reforms that have for decades divided the church’s progressives and conservatives.

Terry didn’t seem likely to be on the front lines of that debate. He studied theater education at Emerson College and then worked in radio, where an executive tried to convince him to change his name to Sandy Beach. In the mid-1980s, one of the radio show’s guests was Providence Mayor Joe Paolino, an old high school pal of Tom’s. The elder Donilon had already gone on to distinguish himself as a White House staffer and one of the Democratic Party’s sharpest political minds. “Come see me,” Paolino told him.

Donilon went to work for Paolino in the mayor’s office and then on his unsuccessful campaign for governor. Then he worked in the office of Gov. Bruce Sundlun and on his unsuccessful reelection campaign. He subsequently worked for Robert Weygand, a member of Congress who lost his bid for a Senate seat. “My candidates kept losing,” Terry, 52, said.

He became press director for a supermarket chain. In 2005, the Boston archdiocese, with the help of a group close to Biden, the patron saint of Donilon family careers, began a search for a new communications director. Donilon said there was no favoritism, and that his interview with O’Malley was going terribly because his beeper kept buzzing and his cellphone kept ringing due to a meat recall by the supermarket chain. Luckily for Donilon, the conversation turned to a mutual acquaintance, former Providence mayor Buddy Cianci (who served jail time and once assaulted his wife’s alleged lover with a lit cigarette, ashtray and fireplace log). That broke the ice.

“I called Tom when I got offered the job,” Terry said, adding that his brother told him, “Don’t be hobnobbing around Boston because you work for an archbishop.”